A hallmark of HIV infection is the recruitment of host factors involved in immune activation that help promote viral replication in various cellular compartments. HIV replication in CD4+ T cells, for example, is most efficient in the activated, memory subset. HIV antigen, in turn, acts to activate HIV-specific cells and, indirectly, can activate non-specific T cells, thus creating a vicious cycle of immune activation and HIV replication. The chronic immune activation generated by persistent HIV antigen is thought to contribute to the immune dysfunction characteristic of HIV disease. This project is designed to further our understanding of the host factors and cellular subsets that influence HIV replication, either directly or by modulating HIV-specific and non-specific immune activation. We are currently investigating the potential role of an immunosuppressive CD4+ T cell subset, CD25+ regulatory T cells (Treg) in the context of HIV infection. We have demonstrated that CD25+ Treg cells inhibit HIV-specific CD4+ and CD8+ T cell immune responses in vitro, particularly in individuals in the early stages of disease. HIV-specific immune responses suppressed by CD25+ Treg cells include proliferation, cytokine and chemokine secretion and the ability of CD8+ T cells to kill HIV expressing target cells. This detrimental effect of CD25+ Treg cells may be counter-balanced by the ability of these cells to suppress generalized, hyper-immune activation (as is induced during HIV infection) thereby reducing the pool of activated CD4+ T cells within which HIV can replicate and also reducing some of the cellular dysfunction and death associated with chronic activation. We have shown that CD25+Treg-like cells can suppress HIV replication in normal CD4+ T cells and are presently assessing whether or not they reduce activation-associated cellular dysfunction. Finally, the activity of lymphoid tissue-associated CD25+ Treg cells in HIV-infected individuals is being determined; this will be particularly relevant as lymphoid tissue is one of the primary sites of HIV replication as well as of the priming of immune responses. A second project investigated the role of host factors in generating and maintaining cellular HIV reservoirs. We have demonstrated that the lymphoid tissue microenvironment supports productive HIV infection in resting CD4+ T cells. Using cultured tonsil tissue, we found that purified resting CD4+ T cells produced HIV mRNA, p24 protein and infectious virus while maintaining a resting, non-dividing T cell phenotype when cultured with autologous lymphoid tissue, but not when cultured alone. HIV production from this cellular population was driven, in part, by pro-inflammatory cytokines and additional signals generated by the tissue matrix. These data repudiate the paradigm that HIV production in T cells requires classical T cell activation and demonstrate that the lymphoid tissue is an ideal microenvironment for suboptimal cellular activation leading to HIV replication in phenotypically resting CD4+ T cells.